1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit including a reflection sheet that contains a fluorescent material sensitive to ultraviolet light.
2. Discussion of the Related Art
Many display devices have been developed and used. Of these devices, cathode ray tubes (CRTs) are mainly used in monitors of television (TV) sets, measuring instruments, information terminals, etc. However, CRTs do not have a small size and are not lightweight. A small size and a light weight are beneficial in electronic products. However, CRTs have a large size and are heavy in weight.
Currently, there is a desire for small and light electronic products, but it is difficult to reduce the weight and size of CRTs. Examples of display devices that are expected to replace CRTs include liquid crystal display (LCD) devices utilizing electro-optic effects, plasma display panels (PDPs) using gas discharge, and electroluminescence displays (ELDs) using electroluminescent effects. Currently, LCD devices are extensively being researched.
LCD devices are substitutes for CRTs because they advantageously have a small size, are lightweight and consume a low amount of power. LCD devices are flat panel display devices, and thus, are widely used in desktop computer monitors, laptop computers and large-scale information displays. Currently, there is a continuously increasing demand for LCD devices.
Because most related art LCD devices are non-emissive devices that regulate the intensity of incident light in order to display pictures and images, they essentially require the use of external light sources, such as backlight units, for irradiating light onto liquid crystal panels.
In most cases, backlight units are used as light sources in LCD devices and are classified into edge-type backlight units and direct-type backlight units. This classification depends on the arrangement of the cylindrical fluorescent lamps in backlight units.
In edge-type backlight units, a lamp unit is positioned at a lateral side of a light guide plate that guides light. The lamp unit includes a light-emitting lamp, a lamp holder inserted between both ends of the lamp to protect the lamp, and a lamp reflection plate surrounding the periphery of the lamp. One side of the lamp reflection plate is fitted into a lateral side of the light guide plate to reflect light emitted from the lamp toward the light guide plate.
Such edge-type backlight units, in which a lamp unit is positioned at a lateral side of a light guide plate, are mostly used in relatively small-sized LCD displays, including monitors of laptop and desktop computers. Edge-type backlight units have the advantages of good light uniformity and long life span. Furthermore, because edge-type backlight units are small, they enable the fabrication of thin LCD devices.
However, as the size of LCD devices is scaled up to 20 inches or above, direct-type backlight units are advantageous and are therefore being actively developed. In direct-type backlight units, a plurality of lamps is arranged in parallel below a diffusion plate so that light is directly irradiated over the entire surface of a liquid crystal panel.
Because direct-type backlight units have higher light availability than edge-type backlight units, they are mostly used in large-screen LCD devices that require high luminance.
LCD devices employing direct-type backlight units are used for a longer time and require a larger number of lamps when they are included in large-sized monitors and television sets as opposed to when they are included in laptop computers. As a result, lamps in direct-type backlight units frequently malfunction and have a short life span compared to the lamps in edge-type backlight units. The possibility that lamps are not properly turned on or off is larger in direct-type LCD devices than in edge-type LCD devices.
Suitable light sources for edge-type and direct-type backlight units include, for example, electroluminescence (EL) lamps, light-emitting diodes (LEDs), cold cathode fluorescent lamps (CCFLs), hot cathode fluorescent lamp (HCFLs), and external electrode fluorescent lamps (EEFLs).
A related art backlight unit will be explained with reference to FIGS. 1 and 2. FIG. 1 is a schematic view showing a related art direct-type backlight unit and FIG. 2 is a perspective view showing a reflection sheet used in the backlight unit of FIG. 1.
As shown in FIG. 1, the related art backlight unit for an LCD device includes a plurality of light-emitting lamps 1, an outer case 3 that fixes and supports the light-emitting lamps 1, and light scattering means 5a, 5b and 5c arranged between the light-emitting lamps 1 and a liquid crystal panel (not shown).
A reflection plate 7 is disposed within the outer case 3 to guide light emitted from the light-emitting lamps 1 toward a display portion of the liquid crystal panel.
The light-emitting lamps 1 may be cold cathode fluorescent lamps (CCFLs). Electrodes are disposed at both ends of a tube of each of the light-emitting lamps 1. When electricity is applied to the electrodes, the lamps begin to emit light. Both ends of the light-emitting lamps 1 fit into holes formed at both sides of the outer case 3.
Electric power leads 9 and 9a are linked to both electrodes of each of the light-emitting lamps 1 to deliver electricity required to drive the lamps 1. The electric power leads 9 and 9a are linked to respective connectors that are connected to a driving circuit. Accordingly, each of the light-emitting lamps 1 necessarily requires a separate connector.
Specifically, the electric power leads 9 and 9a linked to both electrodes of each of the light-emitting lamps 1 are linked to a single connector. One of the electric power leads 9 and 9a is bent below the outer case 3 so as to be linked to the connector.
The related art reflection sheet shown in FIG. 2 includes a colored layer 21, a polyethylene terephthalate (PET) layer 22, a pressure-sensitive adhesive layer 23, and a polyester layer 24. The polyester layer 24 contains an ultraviolet light stabilizer.
The colored layer 21 determines the front and the rear of the reflection sheet. Examples of the UV stabilizer include hydroxybenzophenones, benzotriazoles, substituted acrylates, and hindered amine light stabilizers (HALSs).
Referring to FIG. 2, the reflection sheet serves to diffuse or reflect visible light and absorb ultraviolet light emitted from the light-emitting lamps at the surface of the polyester layer 24.
As a result, the ultraviolet light emitted from the light-emitting lamps cannot be utilized without the reflection sheet. The reflection sheet of the related art direct-type backlight unit is configured to absorb ultraviolet light emitted from the light-emitting lamps, making it impossible to effectively utilize light emitted from the light-emitting lamps. Accordingly, improving the luminance of the backlight unit is limited.
Although a related art direct-type backlight unit equipped with cold cathode fluorescent lamps (CCFLs) has been described, the related art reflection sheet may also be used in direct-type backlight units equipped with external electrode fluorescent lamps (EEFLs). Therefore, improving the luminance of related art direct-type backlight units is limited.